Mammalian mitochondrial succinyl-coenzyme A (CoA) synthetase holoenzyme (SCS; EC 6.2.1.4) is a component of the tricarboxylic acid cycle that catalyzes the formation of succinate from succinyl-CoA and the formation of GTP from GDP and phosphate within the mitochondrial matrix. Succinyl-CoA is the product of NAD.sup.+ -dependant oxidative decarboxylation of .alpha.-ketoglutarate and CoA. In the reverse reaction, succinyl-CoA is decomposed to succinate, CoASH, GDP, and inorganic phosphate by succinyl-CoA ligase (GDP-forming). The SCS holoenzyme is a dimer made up of one .alpha. subunit and one .beta. subunit. (Bridger, W. A. et al. (1987) Biochem. Soc. Symp. 54:103-111.) Sequence analysis of the cDNAs encoding succinyl-CoA synthetase subunits and succinyl-CoA ligase subunits shows that both enzymes are encoded by the same .alpha. subunit and .beta. subunit genes.
Post-translational covalent modification of enzymes can activate or inactivate enzyme activity. SCS activity purified from the slime mold Dictyostelium discoidium is regulated by ATP- and GTP-dependant phosphorylation of the .alpha. subunit. Phosphorylation of the .alpha. subunit is stimulated by low concentrations of GDP, and this appears to correlate with regulation of SCS activity at an allosteric binding site. (Um, H. D. and Klein, C. (1993) 295:821-826.) The .beta. subunit binds CoASH through a disulphide bond to a sulphydryl group of the .beta. subunit. (Collier, G. E. and Nishimura, J. S. (1978) J. Biol. Chem. 253:4938-4943.)
In addition to the activity of SCS in the tricarboxylic acid pathway, SCS activity has been reported in amino acid metabolic pathways. Valine, methionine, and isoleucine are catabolized in a series of enzyme-catalyzed reactions to succinyl-CoA. Biochemical intermediates and enzymes include methacrylyl-CoA, 3-hydroxyisobutyryl-CoA, propionyl CoA carboxylase, hydroxyisobutyric acid, and methylmalonyl CoA mutase. Methylmalonyl CoA mutase activity limits intracellular levels of succinyl-CoA and has been associated with several inherited disorders including methylmalonic aciduria. Methylmalonyl CoA mutase also participates in the oxidation of fatty acids that have an odd number of carbon atoms.
Condensation of succinyl-CoA and glycine to form .delta.-aminolevulinate is the first step of porphyrin biosynthesis. Aberrant succinyl-CoA metabolism may result in clinical symptoms such as dermal photosensitivity and anaemia. Acetazolamide is commonly used in the control of fluid secretion, in particular as an adjunctive treatment for edema due to congestive heart failure, seizures, glaucoma, and to prevent or ameliorate the symptoms associated with acute altitude sickness. SCS activity in the kidney enables ammonia to be excreted in urine, and this activity is inhibited by acetazolamide. (Gougour, A. et al. (1987) Kidney Int. 31:1279-1290.)
The discovery of a new human succinyl-coenzyme A synthetase holoenzyme and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of neoplastic, reproductive, immunological, vesicle trafficking, and nervous disorders.